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High school math: there is no limit to putting three letters in four mailboxes. How many different ways are there?
If there is no limit, there are 4*4*4=64 simple permutation and combination questions.
If each mailbox is limited to 1, there are 4*3*2=24 kinds.
Solution: The first letter has 4 mailboxes to choose from.
Because the first letter uses one mailbox, the second letter has only three mailboxes and the third letter has only two mailboxes.